Tests have been carried out on the synthetic weed and several patients and they came back positive for a lethal ingredient often used in rat poison.

The federal Centers for Disease Control and Prevention have issued a national alert patients with severe, unexplained bleeding may be additional cases.

The CDC is helping Illinois authorities investigate the outbreak in that state, which began in early March. Illinois reported seven more cases on Tuesday, bringing the nationwide total to at least 116.

Doctors across the US are being told to look out for any similar cases (Picture: Getty)

Illnesses have been linked to fake marijuana before but this is the first outbreak involving rat poison contamination, said the CDC’s Renee Funk. Those sickened require hospitalization and treatment with vitamin K to control bleeding, she said.

‘This is an unusual outbreak,’ Funk said Tuesday, adding that is unclear how the contamination occurred.

Fake marijuana, also called synthetic cannabinoids, contains man-made chemicals that produce a high similar to marijuana. It is sold in smoke shops and other stores as liquids that can be used in e-cigarettes or in dried plant material that can be smoked. Nicknames include K2, Spice and Kush.

The outbreak started in March but people are still being admitted to hospitals (Picture: Getty)

‘The number of cases continues to go up each day,’ Melaney Arnold, spokeswoman for the Illinois Department of Public Health, said Tuesday. ‘Synthetic cannabinoids in general are not safe and this is one example of not knowing what chemicals are in the product. We are telling people don’t use synthetic cannabinoids.’

The federal government and many states have banned some of these products or specific ingredients, but the CDC says manufacturers skirt these laws by creating new products or labeling them ‘not for human consumption.’

Tests on fake pot bought at a Chicago convenience store by undercover agents detected the rat poison ingredient, the U.S. attorney’s office said in a news release. Three store employees were arrested and face federal drug charges.

Illnesses from fake marijuana have increased in recent years; a CDC report noted at least 456 cases between 2010 and 2015. The products are up to 100 times more potent than the active ingredient in marijuana and severe reactions have included seizures, coma and delirium.